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Network Working Group P. Vixie
Request for Comments: 2845 ISC
Category: Standards Track O. Gudmundsson
Updates: 1035 NAI Labs
D. Eastlake 3rd
Motorola
B. Wellington
Nominum
May 2000
Secret Key Transaction Authentication for DNS (TSIG)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This protocol allows for transaction level authentication using
shared secrets and one way hashing. It can be used to authenticate
dynamic updates as coming from an approved client, or to authenticate
responses as coming from an approved recursive name server.
No provision has been made here for distributing the shared secrets;
it is expected that a network administrator will statically configure
name servers and clients using some out of band mechanism such as
sneaker-net until a secure automated mechanism for key distribution
is available.
1 - Introduction
1.1. The Domain Name System (DNS) [RFC1034, RFC1035] is a replicated
hierarchical distributed database system that provides information
fundamental to Internet operations, such as name <=> address
translation and mail handling information. DNS has recently been
extended [RFC2535] to provide for data origin authentication, and
public key distribution, all based on public key cryptography and
public key based digital signatures. To be practical, this form of
security generally requires extensive local caching of keys and
tracing of authentication through multiple keys and signatures to a
pre-trusted locally configured key.
1.2. One difficulty with the [RFC2535] scheme is that common DNS
implementations include simple "stub" resolvers which do not have
caches. Such resolvers typically rely on a caching DNS server on
another host. It is impractical for these stub resolvers to perform
general [RFC2535] authentication and they would naturally depend on
their caching DNS server to perform such services for them. To do so
securely requires secure communication of queries and responses.
[RFC2535] provides public key transaction signatures to support this,
but such signatures are very expensive computationally to generate.
In general, these require the same complex public key logic that is
impractical for stubs. This document specifies use of a message
authentication code (MAC), specifically HMAC-MD5 (a keyed hash
function), to provide an efficient means of point-to-point
authentication and integrity checking for transactions.
1.3. A second area where use of straight [RFC2535] public key based
mechanisms may be impractical is authenticating dynamic update
[RFC2136] requests. [RFC2535] provides for request signatures but
with [RFC2535] they, like transaction signatures, require
computationally expensive public key cryptography and complex
authentication logic. Secure Domain Name System Dynamic Update
([RFC2137]) describes how different keys are used in dynamically
updated zones. This document's secret key based MACs can be used to
authenticate DNS update requests as well as transaction responses,
providing a lightweight alternative to the protocol described by
[RFC2137].
1.4. A further use of this mechanism is to protect zone transfers.
In this case the data covered would be the whole zone transfer
including any glue records sent. The protocol described by [RFC2535]
does not protect glue records and unsigned records unless SIG(0)
(transaction signature) is used.
1.5. The authentication mechanism proposed in this document uses
shared secret keys to establish a trust relationship between two
entities. Such keys must be protected in a fashion similar to
private keys, lest a third party masquerade as one of the intended
parties (forge MACs). There is an urgent need to provide simple and
efficient authentication between clients and local servers and this
proposal addresses that need. This proposal is unsuitable for
general server to server authentication for servers which speak with
many other servers, since key management would become unwieldy with
the number of shared keys going up quadratically. But it is suitable
for many resolvers on hosts that only talk to a few recursive
servers.
1.6. A server acting as an indirect caching resolver -- a "forwarder"
in common usage -- might use transaction-based authentication when
communicating with its small number of preconfigured "upstream"
servers. Other uses of DNS secret key authentication and possible
systems for automatic secret key distribution may be proposed in
separate future documents.
1.7. New Assigned Numbers
RRTYPE = TSIG (250)
ERROR = 0..15 (a DNS RCODE)
ERROR = 16 (BADSIG)
ERROR = 17 (BADKEY)
ERROR = 18 (BADTIME)
1.8. The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and
"MAY" in this document are to be interpreted as described in [RFC
2119].
2 - TSIG RR Format
2.1 TSIG RR Type
To provide secret key authentication, we use a new RR type whose
mnemonic is TSIG and whose type code is 250. TSIG is a meta-RR and
MUST not be cached. TSIG RRs are used for authentication between DNS
entities that have established a shared secret key. TSIG RRs are
dynamically computed to cover a particular DNS transaction and are
not DNS RRs in the usual sense.
2.2 TSIG Calculation
As the TSIG RRs are related to one DNS request/response, there is no
value in storing or retransmitting them, thus the TSIG RR is
discarded once it has been used to authenticate a DNS message. The
only message digest algorithm specified in this document is "HMAC-
MD5" (see [RFC1321], [RFC2104]). The "HMAC-MD5" algorithm is
mandatory to implement for interoperability. Other algorithms can be
specified at a later date. Names and definitions of new algorithms
MUST be registered with IANA. All multi-octet integers in the TSIG
record are sent in network byte order (see [RFC1035 2.3.2]).
2.3. Record Format
NAME The name of the key used in domain name syntax. The name
should reflect the names of the hosts and uniquely identify
the key among a set of keys these two hosts may share at any
given time. If hosts A.site.example and B.example.net share a
key, possibilities for the key name include
<id>.A.site.example, <id>.B.example.net, and
<id>.A.site.example.B.example.net. It should be possible for
more than one key to be in simultaneous use among a set of
interacting hosts. The name only needs to be meaningful to
the communicating hosts but a meaningful mnemonic name as
above is strongly recommended.
The name may be used as a local index to the key involved and
it is recommended that it be globally unique. Where a key is
just shared between two hosts, its name actually only need
only be meaningful to them but it is recommended that the key
name be mnemonic and incorporate the resolver and server host
names in that order.
TYPE TSIG (250: Transaction SIGnature)
CLASS ANY
TTL 0
RdLen (variable)
RDATA
Field Name Data Type Notes
--------------------------------------------------------------
Algorithm Name domain-name Name of the algorithm
in domain name syntax.
Time Signed u_int48_t seconds since 1-Jan-70 UTC.
Fudge u_int16_t seconds of error permitted
in Time Signed.
MAC Size u_int16_t number of octets in MAC.
MAC octet stream defined by Algorithm Name.
Original ID u_int16_t original message ID
Error u_int16_t expanded RCODE covering
TSIG processing.
Other Len u_int16_t length, in octets, of
Other Data.
Other Data octet stream empty unless Error == BADTIME
2.4. Example
NAME HOST.EXAMPLE.
TYPE TSIG
CLASS ANY
TTL 0
RdLen as appropriate
RDATA
Field Name Contents
-------------------------------------
Algorithm Name SAMPLE-ALG.EXAMPLE.
Time Signed 853804800
Fudge 300
MAC Size as appropriate
MAC as appropriate
Original ID as appropriate
Error 0 (NOERROR)
Other Len 0
Other Data empty
3 - Protocol Operation
3.1. Effects of adding TSIG to outgoing message
Once the outgoing message has been constructed, the keyed message
digest operation can be performed. The resulting message digest will
then be stored in a TSIG which is appended to the additional data
section (the ARCOUNT is incremented to reflect this). If the TSIG
record cannot be added without causing the message to be truncated,
the server MUST alter the response so that a TSIG can be included.
This response consists of only the question and a TSIG record, and
has the TC bit set and RCODE 0 (NOERROR). The client SHOULD at this
point retry the request using TCP (per [RFC1035 4.2.2]).
3.2. TSIG processing on incoming messages
If an incoming message contains a TSIG record, it MUST be the last
record in the additional section. Multiple TSIG records are not
allowed. If a TSIG record is present in any other position, the
packet is dropped and a response with RCODE 1 (FORMERR) MUST be
returned. Upon receipt of a message with a correctly placed TSIG RR,
the TSIG RR is copied to a safe location, removed from the DNS
Message, and decremented out of the DNS message header's ARCOUNT. At
this point the keyed message digest operation is performed. If the
algorithm name or key name is unknown to the recipient, or if the
message digests do not match, the whole DNS message MUST be
discarded. If the message is a query, a response with RCODE 9
(NOTAUTH) MUST be sent back to the originator with TSIG ERROR 17
(BADKEY) or TSIG ERROR 16 (BADSIG). If no key is available to sign
this message it MUST be sent unsigned (MAC size == 0 and empty MAC).
A message to the system operations log SHOULD be generated, to warn
the operations staff of a possible security incident in progress.
Care should be taken to ensure that logging of this type of event
does not open the system to a denial of service attack.
3.3. Time values used in TSIG calculations
The data digested includes the two timer values in the TSIG header in
order to defend against replay attacks. If this were not done, an
attacker could replay old messages but update the "Time Signed" and
"Fudge" fields to make the message look new. This data is named
"TSIG Timers", and for the purpose of digest calculation they are
invoked in their "on the wire" format, in the following order: first
Time Signed, then Fudge. For example:
Field Name Value Wire Format Meaning
----------------------------------------------------------------------
Time Signed 853804800 00 00 32 e4 07 00 Tue Jan 21 00:00:00 1997
Fudge 300 01 2C 5 minutes
3.4. TSIG Variables and Coverage
When generating or verifying the contents of a TSIG record, the
following data are digested, in network byte order or wire format, as
appropriate:
3.4.1. DNS Message
A whole and complete DNS message in wire format, before the TSIG RR
has been added to the additional data section and before the DNS
Message Header's ARCOUNT field has been incremented to contain the
TSIG RR. If the message ID differs from the original message ID, the
original message ID is substituted for the message ID. This could
happen when forwarding a dynamic update request, for example.
3.4.2. TSIG Variables
Source Field Name Notes
-----------------------------------------------------------------------
TSIG RR NAME Key name, in canonical wire format
TSIG RR CLASS (Always ANY in the current specification)
TSIG RR TTL (Always 0 in the current specification)
TSIG RDATA Algorithm Name in canonical wire format
TSIG RDATA Time Signed in network byte order
TSIG RDATA Fudge in network byte order
TSIG RDATA Error in network byte order
TSIG RDATA Other Len in network byte order
TSIG RDATA Other Data exactly as transmitted
The RR RDLEN and RDATA MAC Length are not included in the hash since
they are not guaranteed to be knowable before the MAC is generated.
The Original ID field is not included in this section, as it has
already been substituted for the message ID in the DNS header and
hashed.
For each label type, there must be a defined "Canonical wire format"
that specifies how to express a label in an unambiguous way. For
label type 00, this is defined in [RFC2535], for label type 01, this
is defined in [RFC2673]. The use of label types other than 00 and 01
is not defined for this specification.
3.4.3. Request MAC
When generating the MAC to be included in a response, the request MAC
must be included in the digest. The request's MAC is digested in
wire format, including the following fields:
Field Type Description
---------------------------------------------------
MAC Length u_int16_t in network byte order
MAC Data octet stream exactly as transmitted
3.5. Padding
Digested components are fed into the hashing function as a continuous
octet stream with no interfield padding.
4 - Protocol Details
4.1. TSIG generation on requests
Client performs the message digest operation and appends a TSIG
record to the additional data section and transmits the request to
the server. The client MUST store the message digest from the
request while awaiting an answer. The digest components for a
request are:
DNS Message (request)
TSIG Variables (request)
Note that some older name servers will not accept requests with a
nonempty additional data section. Clients SHOULD only attempt signed
transactions with servers who are known to support TSIG and share
some secret key with the client -- so, this is not a problem in
practice.
4.2. TSIG on Answers
When a server has generated a response to a signed request, it signs
the response using the same algorithm and key. The server MUST not
generate a signed response to an unsigned request. The digest
components are:
Request MAC
DNS Message (response)
TSIG Variables (response)
4.3. TSIG on TSIG Error returns
When a server detects an error relating to the key or MAC, the server
SHOULD send back an unsigned error message (MAC size == 0 and empty
MAC). If an error is detected relating to the TSIG validity period,
the server SHOULD send back a signed error message. The digest
components are:
Request MAC (if the request MAC validated)
DNS Message (response)
TSIG Variables (response)
The reason that the request is not included in this digest in some
cases is to make it possible for the client to verify the error. If
the error is not a TSIG error the response MUST be generated as
specified in [4.2].
4.4. TSIG on TCP connection
A DNS TCP session can include multiple DNS envelopes. This is, for
example, commonly used by zone transfer. Using TSIG on such a
connection can protect the connection from hijacking and provide data
integrity. The TSIG MUST be included on the first and last DNS
envelopes. It can be optionally placed on any intermediary
envelopes. It is expensive to include it on every envelopes, but it
MUST be placed on at least every 100'th envelope. The first envelope
is processed as a standard answer, and subsequent messages have the
following digest components:
Prior Digest (running)
DNS Messages (any unsigned messages since the last TSIG)
TSIG Timers (current message)
This allows the client to rapidly detect when the session has been
altered; at which point it can close the connection and retry. If a
client TSIG verification fails, the client MUST close the connection.
If the client does not receive TSIG records frequently enough (as
specified above) it SHOULD assume the connection has been hijacked
and it SHOULD close the connection. The client SHOULD treat this the
same way as they would any other interrupted transfer (although the
exact behavior is not specified).
4.5. Server TSIG checks
Upon receipt of a message, server will check if there is a TSIG RR.
If one exists, the server is REQUIRED to return a TSIG RR in the
response. The server MUST perform the following checks in the
following order, check KEY, check TIME values, check MAC.
4.5.1. KEY check and error handling
If a non-forwarding server does not recognize the key used by the
client, the server MUST generate an error response with RCODE 9
(NOTAUTH) and TSIG ERROR 17 (BADKEY). This response MUST be unsigned
as specified in [4.3]. The server SHOULD log the error.
4.5.2. TIME check and error handling
If the server time is outside the time interval specified by the
request (which is: Time Signed, plus/minus Fudge), the server MUST
generate an error response with RCODE 9 (NOTAUTH) and TSIG ERROR 18
(BADTIME). The server SHOULD also cache the most recent time signed
value in a message generated by a key, and SHOULD return BADTIME if a
message received later has an earlier time signed value. A response
indicating a BADTIME error MUST be signed by the same key as the
request. It MUST include the client's current time in the time
signed field, the server's current time (a u_int48_t) in the other
data field, and 6 in the other data length field. This is done so
that the client can verify a message with a BADTIME error without the
verification failing due to another BADTIME error. The data signed
is specified in [4.3]. The server SHOULD log the error.
4.5.3. MAC check and error handling
If a TSIG fails to verify, the server MUST generate an error response
as specified in [4.3] with RCODE 9 (NOTAUTH) and TSIG ERROR 16
(BADSIG). This response MUST be unsigned as specified in [4.3]. The
server SHOULD log the error.
4.6. Client processing of answer
When a client receives a response from a server and expects to see a
TSIG, it first checks if the TSIG RR is present in the response.
Otherwise, the response is treated as having a format error and
discarded. The client then extracts the TSIG, adjusts the ARCOUNT,
and calculates the keyed digest in the same way as the server. If
the TSIG does not validate, that response MUST be discarded, unless
the RCODE is 9 (NOTAUTH), in which case the client SHOULD attempt to
verify the response as if it were a TSIG Error response, as specified
in [4.3]. A message containing an unsigned TSIG record or a TSIG
record which fails verification SHOULD not be considered an
acceptable response; the client SHOULD log an error and continue to
wait for a signed response until the request times out.
4.6.1. Key error handling
If an RCODE on a response is 9 (NOTAUTH), and the response TSIG
validates, and the TSIG key is different from the key used on the
request, then this is a KEY error. The client MAY retry the request
using the key specified by the server. This should never occur, as a
server MUST NOT sign a response with a different key than signed the
request.
4.6.2. Time error handling
If the response RCODE is 9 (NOTAUTH) and the TSIG ERROR is 18
(BADTIME), or the current time does not fall in the range specified
in the TSIG record, then this is a TIME error. This is an indication
that the client and server clocks are not synchronized. In this case
the client SHOULD log the event. DNS resolvers MUST NOT adjust any
clocks in the client based on BADTIME errors, but the server's time
in the other data field SHOULD be logged.
4.6.3. MAC error handling
If the response RCODE is 9 (NOTAUTH) and TSIG ERROR is 16 (BADSIG),
this is a MAC error, and client MAY retry the request with a new
request ID but it would be better to try a different shared key if
one is available. Client SHOULD keep track of how many MAC errors
are associated with each key. Clients SHOULD log this event.
4.7. Special considerations for forwarding servers
A server acting as a forwarding server of a DNS message SHOULD check
for the existence of a TSIG record. If the name on the TSIG is not
of a secret that the server shares with the originator the server
MUST forward the message unchanged including the TSIG. If the name
of the TSIG is of a key this server shares with the originator, it
MUST process the TSIG. If the TSIG passes all checks, the forwarding
server MUST, if possible, include a TSIG of his own, to the
destination or the next forwarder. If no transaction security is
available to the destination and the response has the AD flag (see
[RFC2535]), the forwarder MUST unset the AD flag before adding the
TSIG to the answer.
5 - Shared Secrets
5.1. Secret keys are very sensitive information and all available
steps should be taken to protect them on every host on which they are
stored. Generally such hosts need to be physically protected. If
they are multi-user machines, great care should be taken that
unprivileged users have no access to keying material. Resolvers
often run unprivileged, which means all users of a host would be able
to see whatever configuration data is used by the resolver.
5.2. A name server usually runs privileged, which means its
configuration data need not be visible to all users of the host. For
this reason, a host that implements transaction-based authentication
should probably be configured with a "stub resolver" and a local
caching and forwarding name server. This presents a special problem
for [RFC2136] which otherwise depends on clients to communicate only
with a zone's authoritative name servers.
5.3. Use of strong random shared secrets is essential to the security
of TSIG. See [RFC1750] for a discussion of this issue. The secret
should be at least as long as the keyed message digest, i.e. 16 bytes
for HMAC-MD5 or 20 bytes for HMAC-SHA1.
6 - Security Considerations
6.1. The approach specified here is computationally much less
expensive than the signatures specified in [RFC2535]. As long as the
shared secret key is not compromised, strong authentication is
provided for the last hop from a local name server to the user
resolver.
6.2. Secret keys should be changed periodically. If the client host
has been compromised, the server should suspend the use of all
secrets known to that client. If possible, secrets should be stored
in encrypted form. Secrets should never be transmitted in the clear
over any network. This document does not address the issue on how to
distribute secrets. Secrets should never be shared by more than two
entities.
6.3. This mechanism does not authenticate source data, only its
transmission between two parties who share some secret. The original
source data can come from a compromised zone master or can be
corrupted during transit from an authentic zone master to some
"caching forwarder." However, if the server is faithfully performing
the full [RFC2535] security checks, then only security checked data
will be available to the client.
6.4. A fudge value that is too large may leave the server open to
replay attacks. A fudge value that is too small may cause failures
if machines are not time synchronized or there are unexpected network
delays. The recommended value in most situation is 300 seconds.
7 - IANA Considerations
IANA is expected to create and maintain a registry of algorithm names
to be used as "Algorithm Names" as defined in Section 2.3. The
initial value should be "HMAC-MD5.SIG-ALG.REG.INT". Algorithm names
are text strings encoded using the syntax of a domain name. There is
no structure required other than names for different algorithms must
be unique when compared as DNS names, i.e., comparison is case
insensitive. Note that the initial value mentioned above is not a
domain name, and therefore need not be a registered name within the
DNS. New algorithms are assigned using the IETF Consensus policy
defined in RFC 2434. The algorithm name HMAC-MD5.SIG-ALG.REG.INT
looks like a FQDN for historical reasons; future algorithm names are
expected to be simple (i.e., single-component) names.
IANA is expected to create and maintain a registry of "TSIG Error
values" to be used for "Error" values as defined in section 2.3.
Initial values should be those defined in section 1.7. New TSIG
error codes for the TSIG error field are assigned using the IETF
Consensus policy defined in RFC 2434.
8 - References
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1034, November 1987.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1995.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC-MD5:
Keyed-MD5 for Message Authentication", RFC 2104, February
1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound "Dynamic
Updates in the Domain Name System", RFC 2136, April 1997.
[RFC2137] Eastlake 3rd, D., "Secure Domain Name System Dynamic
Update", RFC 2137, April 1997.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2673] Crawford, M., "Binary Labels in the Domain Name System",
RFC 2673, August 1999.
9 - Authors' Addresses
Paul Vixie
Internet Software Consortium
950 Charter Street
Redwood City, CA 94063
Phone: +1 650 779 7001
EMail: vixie@isc.org
Olafur Gudmundsson
NAI Labs
3060 Washington Road, Route 97
Glenwood, MD 21738
Phone: +1 443 259 2389
EMail: ogud@tislabs.com
Donald E. Eastlake 3rd
Motorola
140 Forest Avenue
Hudson, MA 01749 USA
Phone: +1 508 261 5434
EMail: dee3@torque.pothole.com
Brian Wellington
Nominum, Inc.
950 Charter Street
Redwood City, CA 94063
Phone: +1 650 779 6022
EMail: Brian.Wellington@nominum.com
10 Full Copyright Statement
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Acknowledgement
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